Arrangement and method for monitoring of annulus volume

ABSTRACT

The present description is related to integrity monitoring of an annulus volume in a pipe. More specifically, the present disclosure is related to an arrangement and a method for determination of annulus free volume of a pipe. The monitoring can as an example be performed by use of a logical unit which controls the annulus testing by use of diffusion or gas feed, and also give input to calculation of the pipe&#39;s remaining lifetime. Further, the arrangement can as an example save and present results, allow for user specified settings and set off alarms it critical values are detected.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of International Application No.PCT/NO2015/050127 filed on Jul. 8, 2015. Priority is claimed fromNorwegian Patent Application No. 20140864 filed on Jul. 8, 2014.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not Applicable.

BACKGROUND

The present disclosure is related to an arrangement and a method forannulus volume integrity monitoring. More specifically the presentdisclosure relates to an arrangement and method for determination ofannulus free volume of a flexible riser. The monitoring can be performedby use of a logical unit which runs the annulus testing with diffusionor gas feed.

Monitoring and integrity evaluation are performed to control and map thecondition and quality of the annulus volume in as an example a pipe,typically a flexible riser. Annulus testing for calculation of annulusfree volume is a significant part of the pipe integrity evaluation. Asshown in FIG. 1, the annulus volume 102 in a flexible pipe is defined asthe area between the outer sheath 101 and the pressure barrier 103.

Based on such an annulus test, the fluid content in annulus can beevaluated by comparing the measured annulus free volume and knowledge ofthe total annulus volume. The annulus volume in a flexible pipe isexpected to remain dry or experience a slow filling during time due toamong other factors diffusion through the pipe pressure barrier. Annulustesting of flexible risers is per today typically performed annually atoffshore installations in the Norwegian continental shelf. FIG. 2 showsan example sketch of an offshore installation which comprises a flexiblepipe 201, a platform 202 and a test location 203. The annulus testing ofthe flexible pipe 201 is typically performed at test location 203.

Results from manual annulus tests are typically input for calculation ofthe pipe's remaining lifetime. As an example the annulus volume can befluid filled due to damaged outer sheath. The deterioration andtherefore the remaining pipe lifetime with fluid filled annulus, istypically calculated based on the time of execution of the latestannulus test. If latest test was performed one year ago, one year of theremaining lifetime must be subtracted, by assuming that the damageappeared shortly after the previous test. Large expenses are thereforelikely to occur by not being able to operate the pipe up to one moreyear or due to short time for planning when a flexible pipe needs to bereplaced. The long time intervals between each annulus test is thereforea problem today. Especially some pipes require tight follow-up.

At present, annulus testing is performed manually, which requires keypersonal offshore to perform the testing. Thus the testing is dependenton human variations and factors, which can result in tests not beingperformed or not to be performed when scheduled. Additionally, humanfactors can cause separate parameter basis for separate tests. It canalso be mentioned that for some pipes ventilating of annulus isperformed directly in to the open, which cause danger for corrosion dueto oxygen access.

SUMMARY

According to the present disclosure, the above mentioned problems may besolved by an arrangement and method for determination of a pipe'sannulus free volume and monitoring of the annulus volume state.

The mentioned monitoring can comprise initiation of pressure build up byadjusting the operational pressure to a predefined high pressure limit,keep the pressure for a given time period for the pressure to bestabilized, decide whether this high pressure is stabilized and performat least one measurement of the pressure. If the pressure is found to bewithin a given accuracy compared to the predefined high pressure limit,the operation pressure can be adjusted to a predefined low pressurelimit. Depressurization is obtained by emptying the reference volume atleast one time. Keep the pressure for a given period of time for thepressure to stabilize, decide whether the pressure is stabilized at thelow pressure limit and perform at least on measurement of the pressure.If the pressure is found to be within a given accuracy compared to thepredefined low pressure limit, the accumulated volume can be calculatedbased on data from the emptying of the at least one reference volume.

A first aspect of the present disclosure is an arrangement fordetermination of a pipe's annulus free volume, where the arrangementincludes a reference volume, a valve connected to the annulus freevolume and to the reference volume for depressurizing of the annulusvolume to the reference volume, at least one valve connected to thereference volume and at least one outlet for depressurizing of thereference volume, and at least on pressure instrument for measurement ofthe annulus volume pressure, where the mentioned valves and the at leastone pressure instrument is arranged to be read off and controlled forpressurizing and depressurizing, and where measurements of referencevolume pressure and annulus volume pressure prior to and afterdepressurizing and the reference volume dimensions are used forcalculation of the annulus free volume.

Calculation of the annulus free volume is performed by use of equationsdescribed below.

The arrangement can further comprise at least one logical unit for readoff and control of the mentioned valves and the at least one pressureinstrument during pressurizing and depressurizing. The logical unit isfurther arranged to use the pressure measurements in the referencevolume and the annulus volume prior to and after depressurization andthe reference volume dimension/size for calculation of the annulus freevolume.

The arrangement can further comprise at least one gas container forpressurizing, one pressure regulator connected to the gas container toregulate the gas feed, and at least one valve to open and close forpressurization of the at least one annulus volume.

Further, the arrangement can comprise a pressure instrument connected tothe gas container to gas container pressure measurement, and at leastone pressure instrument connected to the pressure regulator forindication of the regulator settings. The mentioned pressure instrumentsare further connected to the logical unit for controlling and read off.

Additionally, the arrangement can comprise at least one pressureinstrument for measurement of the reference volume pressure. Thearrangement can comprise at least one pressure relief valve to controlthe deflation from the gas container, and eventually at least onepressure relief valve to control the deflation from the at least oneannulus volume directly through the outlet. The gas feed is given fromthe mentioned external gas container or an internal gas container.

Another aspect of the present disclosure is a method for determinationof a annulus free volume in a pipe, where a pressure difference isestablished between the annulus volume and the reference volume, anumber of depressurizations from the annulus volume to the referencevolume, where the number is one or more, pressure is measured prior toand after the number of depressurizations. The annulus free volume isthen calculated based on pressure measurements in the annulus volumeprior to and after the number of depressurizations, together withpressure measurements for each depressurization in the reference volumeand the reference volume dimension/size.

The method can comprise steps for establishment of a predefinedreference volume pressure prior to each depressurization, and referencevolume pressure measurement after each depressurization.

Further, the pressure difference establishment can be performed bydiffused gas or by applying gas from a gas container to the annulusvolume.

The method can comprise steps to decide whether measured pressure in theannulus volume has reached the high pressure limit, followed by keepingthis established pressure for a given period of time if necessary, anddecide whether measured pressure in the annulus volume after the sameperiod of time is within a given accuracy of the high pressure limit,further establishment of a pressure difference if this is not the case,and thereafter followed by the steps after the depressurizing asdescribed above.

Further, the method can comprise decision of whether the measuredpressure have reached the low pressure limit after depressurizing,followed by keeping this pressure for a given period of time if this isthe case, decision of whether measured pressure in the annulus volumeafter this period of time is within a given accuracy to the low pressurelimit, followed by further depressurizing if this is not the case, andthereafter followed by steps for calculation of annulus free volume.

Calculation of the annulus free volume can be performed by use of thefollowing equations:

$V = \frac{V_{out}}{\left( {P_{a\; 1} - P_{a\; 2}} \right)}$$V_{at} = {\sum\limits_{i = 1}^{i = n}\; {V_{ref}\left( {P_{{ref}\; 1i} - P_{{ref}\; 2i}} \right)}}$

Where:

-   -   V is annulus free volume,    -   V_(out) is accumulated volume during depressurization,    -   V_(ref) is reference volume,    -   P_(a1) is annulus pressure prior to the number of        depressurizations (i=1)    -   P_(a2) is annulus pressure after the number of depressurizations        (i=n)    -   P_(ref1) is pressure in the reference volume prior to        depressurization number i to the outlet,    -   P_(ref2) is pressure in the reference volume after        depressurization number i to the outlet, and    -   n is the number of depressurization cycles.

The accumulated volume calculation can be performed by at least onelogical unit based on data received when emptying at least one referencevolume by use of a valve at least one time until a low pressure limit inthe annulus volume is reached.

The pressure difference establishment can comprise pressurizationperformed by closing a valve if diffusion is used, or by adjustment ofpressure regulator, and opening of the valve to the gas container andclosing of the valve to the reference volume if the pressurization isperformed by use of gas feed.

The method can comprise decision of whether the pressure is stabilizedat the high pressure limit, by pressure measurement for diffused gas, orby closing of valve and pressure measurement if gas feed is used.

Emptying the reference volume can be performed at least one time byclosing the valve in-between the annulus volume and the referencevolume, and by opening the valve in-between the reference volume and thebelonging outlet, until the low pressure limit in the annulus volume isreached.

Decision of whether or not the pressure is stabilized, can be performedby pressure measurement by use of a pressure instrument arranged at theannulus volume or at the reference volume.

The method can comprise volume calculations which allows for correctionfor diffusion and temperature differences, and control of one or morecomponents of an arrangement as described above. The logical unit canperform such calculations and corrections.

The method can comprise the following features performed by the logicalunit: control of at least one valve for pressurizing and depressurizingof at least one annulus volume, read off of at least one pressureinstrument for pressure measurement of at least one reference volume,control of at least one valve for pressurizing and depressurizing of atleast one reference volume, and read off of at least one pressureinstrument for pressure measurement of at least one annulus volume.

Further, the following features of the method can be performed by alogical unit: read off of at least one pressure instrument for pressuremeasurement of at least one gas container, control of at least onepressure regulator for pressure adjustment of the gas feed from the gascontainer, read off of at least one pressure instrument to control theadjustments of the pressure regulator, and control of at least one valvefor depressurizing of at least one gas container.

A third aspect of the present disclosure is a method for monitoring ofthe annulus volume integrity comprising a method for calculation ofannulus free volume as described above, where the method comprises oneor more of the following steps: comparing the new annulus free volume toone or more previous calculated annulus free volumes, comparingaccumulated volume per time to a normal condition, comparing accumulatedvolume per time to an historical trend, use of annulus free volume forcalculation of pipe's remaining lifetime, saving and presenting measuredand calculated values, and triggering one or more alarms if criticalvalues are found. The method can be performed continuously or at regularintervals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the cross section of a flexible pipe.

FIG. 2 shows an example of an offshore installation which comprises aflexible pipe.

FIG. 3 shows an arrangement for monitoring of the integrity of at leastone annulus volume based on diffused gas.

FIG. 4 shows an arrangement for monitoring of the integrity of at leastone annulus volume based on gas feed.

FIG. 5 shows an example flow chart for the method of integritymonitoring of at least one annulus volume.

All figures are schematic and not in scale, and they are showing onlythe parts necessary to illustrate the arrangement and method, otherparts are omitted or merely indicated.

DETAILED DESCRIPTION

The solution according to the present disclosure may be obtained by amethod and an arrangement in accordance to the characterizing parts ofthe independent claims.

The present disclosure is related to an arrangement and a method formonitoring of the annulus volume integrity. More specifically, thepresent disclosure relates to an arrangement and a method fordetermination of an annulus free volume in a pipe. As an example themonitoring can be performed by use of an logical unit which controls theannulus testing by diffusion or gas feed, and, based on the testresults, calculates the annulus free volume. The test results can as anexample be input to calculation of the pipe remaining lifetime.

The annulus testing can in a first embodiment comprise use of diffusedgas and in a second embodiment comprise use of gas feed. By use ofdiffused gas, the mentioned monitoring is performed by waiting for anatural pressurization of the annulus volume. This is well suited forhigh diffusion pipes. However, for flexible pipes with low or nodiffusion, gas feed is typically used. The advantage of gas feed, as anexample nitrogen, is that the annulus testing will typically be lesstime consuming. The advantage of use of diffused gas is that thearrangement or product consist of fewer components and is thus lessexposed to errors and more automotive.

In a first embodiment, i.e. the test setup for annulus testing by use ofdiffused gas, the arrangement typically comprises, not limited by, thecomponents inside box 309 in FIG. 3; at least one valve 302, as anexample an solenoid valve, connected to at least one annulus volume 301,at least one pressure instrument 303 for pressure measurement of atleast one reference volume container 304, also called reference volume,the at least one reference volume 304 connected to a pressure instrument303 and valve 302, at least one valve 305 connected to the at least onereference volume 304 and at least one outlet 306, at least one pressurerelief valve 307 which for safety issues is directly connected from atthe least one annulus volume 301 to the at least one outlet 306, atleast one pressure instrument 316 for pressure measurement of theannulus volume 301, and at least one logical unit 308 connected to,controlling and reading off pressure instrument 316, valve 302, pressureinstrument 303 and valve 305. The arrangement 309 can, based on theabove, monitor one or more annulus volumes 301 and FIG. 3 thus shows amonitoring system.

In the illustrated embodiment a pressure instrument 303, 316 is arrangedat both the annulus volume 301 and at the reference volume 304. Inanother embodiment one single pressure instrument 303 or 316 can be usedfor measuring of the annulus volume or the reference volume.

In a second embodiment, test setup for annulus testing by use of gasfeed, the arrangement typically comprises, not limited by, thecomponents inside box 409 in FIG. 4. In addition to the componentsdescribed in the first embodiment, this arrangement can comprise atleast one gas container 410, where gas container 410 can be locatedinternally or externally the arrangement 409, i.e. the product. FIG. 4show a gas container 410 as an external gas container. Morespecifically, the mentioned arrangement comprises at least one pressureinstrument 411 connected to at least one gas container 410, used tomeasure the remaining gas in the gas container 410, at least onepressure regulator 412, two-stages or more accurate regulator, connectedto at least one gas container 410, at least one pressure release valve413 connected to pressure regulator 412, at least one pressureinstrument 414, for read off of the pressure regulator 412 settings, atleast one valve 415 connected to a pressure regulator 412 and thus thegas container 410, used for pressurizing, at least one pressureinstrument 416 for pressure measurement of the annulus volume 401, atleast one valve 402 connected to at least one annulus volume 401, atleast one pressure instrument 403 for pressure measurements of thereference volume 404, at least one valve 405 connected to at least onereference volume 404 and at least one outlet 406, at least one pressurerelief valve 407 directly connected from the annulus volume 401 to atleast one outlet 406 for safety, and finally, at least one logical unit408 connected to, controlling and reading off valve 402, pressureinstrument 403, valve 405, pressure instrument 416, pressure instrument411, pressure regulator 412, pressure instrument 414 and valve 415.Additionally, valve 415 is connected to the annulus volume 401. Thearrangement 409 can be used for monitoring of one or more annulusvolumes 401 and thus FIG. 4 shows a monitoring system.

For both the above mentioned embodiments the logical unit 308, 408contains software for controlling of, as an example, pressureinstruments and valves. The arrangement 309, 409 is therefore able tomeasure annulus volume 301, 401 pressure, control valves forpressurizing and depressurizing, and also perform calculations. Further,the monitoring system is able to save and present results, use userspecified settings, give one or more alarms if values outside the normalrange is seen and issue the results to external systems.

The pressure instruments 305, 405, 316, 416 can measure pressure both inthe annulus volume 301, 401 and in the reference volume 304, 404, whenvalve 302, 402 in-between the mentioned volumes is open.

The pressure difference between the annulus volume 301, 401 and thereference volume 304, 404 can as an example be obtained by an annuluspressure above atmospheric pressure and then typically with atmosphericpressure in the reference volume and the outlet 306, 406, or by vacuumat the outlet and thus in the reference volume 304, 404.

Typically, a relatively small reference volume 304, 404 will be used, itcould be one or more magnitudes smaller than the annulus volume 301,401, and emptied several times. However, the arrangement and methodaccording to the present disclosure are also applicable for a relativelylarge reference volume 304, 404, which can be same size or even largerthan the annulus volume 301, 401, and emptied few times.

In another embodiment of the present disclosure the arrangement 309, 409can, in-between the annulus testing, be used for diffusion flow ratemeasurement of the annulus volume 301, 401. The diffusion flow rate canalso be utilized for calibration of the calculated annulus volume.

In an even further embodiment according to the present disclosure, thearrangement 309, 409 can be a part of a larger monitoring system, usedfor continuous or regular integrity monitoring of the annulus volume,and in an even further embodiment the mentioned monitoring system can bepart of a control system for control of one or more flows and gas orfluid pressures in annulus volumes in, as an example, pipes.

In an embodiment according to the present disclosure the arrangement309, 409 is controlled by use of a logical unit 308, 408. However, it ispossible in some embodiments to control the arrangement componentsmanually.

At least one annulus test is performed when monitoring the annulusvolume integrity. As an example, the annulus testing can comprise stepslike shown in FIG. 5. In the first step, the arrangement is set toperform an annulus test, i.e. the arrangement is set to annulus testingmode 501. In the next step the arrangement is initiating apressurization 502. If necessary the pressure is kept for a given periodof time until it is stabilized 503. Thereafter, the arrangement isperforming a pressure measurement to check whether the high pressurelimit is reached. If not, a new pressurization is performed 502. If thehigh pressure limit is reached, the process continues to step 504.Thereafter a depressurization is performed 505. If necessary, thepressure is kept for period of time until it is stabilized 506. Thearrangement is thereafter performing a new pressure measurement to checkwhether the low pressure limit is reached. If not, a newdepressurization is performed 505. If the low pressure limit is reached,the process continues to step 507. Thereafter annulus volumecalculations are performed 508. The annulus testing is in this exampleended by setting the arrangement back to standard mode 509.

With reference to the annulus test example above, it is emphasized thatin other embodiments a standard mode is not needed. The purpose of thesteps related to keeping the pressure at a high and low pressure limitis to increase the accuracy and is not needed in other embodiments.

An example embodiment of the method for monitoring of the annulusintegrity is described more in detail in the following. The stepscomprise pressurization both by use of diffusion and gas feed.

The method steps can as an example be:

-   -   1) Initiate pressurization for pressure build up to a predefined        high pressure limit, by use of diffusion or gas feed. Often a        minimum pressurization period is used.        -   Diffusion: valve 302 open and valve 305 closed        -   Gas feed: valve 402 and valve 415 open and valve 405 closed,            the pressure regulator 412 is set to the high pressure limit    -   2) When the annulus pressure reaches the high pressure limit,        keep this pressure for a given period of time.        -   Diffusion: time period zero        -   Gas feed: valve 415 closed, arrangement need to keep the            pressure for a given period of time for the pressure to            stabilize    -   3) Pressure measurement after stabilizing. If the measured        pressure is within a given accuracy to the high pressure limit,        as an example 90 to 110% of the high pressure limit, continue to        next step. If not, perform a new pressurization (i.e. start at        step 1)).        -   Diffusion: pressure instrument 303 is used for pressure            measurement        -   Gas feed: valve 402 open, valve 405 and 415 closed, pressure            instrument 403 is used for pressure measurement    -   4) Initiate depressurization to a predefined low pressure limit.        Flow measurement cycles will then be run by the arrangement to        meet the new operational settings. Accumulated volume is        calculated by the arrangement by emptying the annulus volume        through the reference volume multiple times to meet the low        pressure limit.        -   Diffusion: valve 302 open, valve 305 closed, thereafter            emptying reference volume by closing valve 302 and opening            valve 305. Repeat this step until the low pressure limit is            reached        -   Gas feed: valve 405 and 415 closed, valve 402 open,            thereafter emptying reference volume by closing valve 402            and opening valve 405. Repeat this step until the low            pressure limit is reached    -   5) When the pressure is within a given accuracy to the low        pressure limit, as an example 90 to 110% of the low pressure        limit, the arrangement will keep the pressure for a given period        of time for the pressure to stabilize.        -   Diffusion: time period zero        -   Gas feed: arrangement need to keep the pressure a given            period of time for the pressure to stabilize    -   6) Pressure measurement after stabilizing. If the pressure is        measured to be within a given accuracy to the low pressure        limit, continue to next step. If not, perform a new        depressurization (i.e. start at step 4)).        -   Diffusion: pressure instrument 303 is used for pressure            measurement        -   Gas feed: pressure instrument 403 is used for pressure            measurement    -   7) When measurements are ended, the annulus free volume is        calculated by the logical unit 308, 408.    -   8) Test ended.

According to some embodiments of the present disclosure the annulus freevolume is calculated by use of the following equations.

Volume calculations are performed based on the (combined) ideal gasequation:

$\frac{PV}{T} = C$

Where P is absolute pressure, V is volume, T is absolute temperature(Kelvin) and C is a constant.

The calculation of annulus free volume can be based on the followingequations:

$V = \frac{V_{out}}{\left( {P_{a\; 1} - P_{a\; 2}} \right)}$$V_{at} = {\sum\limits_{i = 1}^{i = n}\; {V_{ref}\left( {P_{{ref}\; 1i} - P_{{ref}\; 2i}} \right)}}$

Where:

-   -   V is annulus free volume,    -   V_(out) is accumulated volume during depressurization,    -   V_(ref) is reference volume,    -   P_(a1) is annulus pressure prior to the number of        depressurizations (i=1)    -   P_(a2) is annulus pressure after the number of depressurizations        (i=n)    -   P_(ref1) is pressure in the reference volume prior to        depressurization number i to the outlet,    -   P_(ref2) is pressure in the reference volume after        depressurization number i to the outlet, and    -   n is the number of depressurization cycles.

The equations above are based on a case where the temperature is assumedto be constant and equal inside and outside the annulus volume 301, 401.

As previously mentioned, the accumulated volume can be calculated byemptying the reference volume 304, 404 repeatedly until a low pressurelimit in the annulus volume 301, 401 is reached.

The reference volume pressure after emptying can as an example be equalto atmospheric pressure or be vacuum.

The annulus test results are typically used as input for calculation ofthe pipe's remaining lifetime. As an example, this could be a pipe wherethe annulus is fluid filled due to outer sheath damage.

It can further be mentioned that the present disclosure is a one waysystem, which ensures no oxygen into the annulus volume.

Even though the present disclosure is described with reference to alimited number of embodiments, a person of ordinary skill in the art,having the benefit of the present disclosure will understand thatmodifications are possible with no deviation from the scope of thepresent disclosure as it is defined in the following claims.

What is claimed is:
 1. An arrangement for determination of annulus freevolume (301, 401) in a pipe, the arrangement comprising: a referencevolume (304, 404), at least one valve (302, 402) connected to theannulus volume (301, 401) and the reference volume (304, 404) fordepressurization of the annulus volume (301, 401) to the referencevolume (304, 404), at least one valve (305, 405) connected to thereference volume (304, 404) and at least one outlet (306, 406) fordepressurization of the reference volume (304, 404), and at least onepressure instrument (316, 416, 303, 403) for pressure measurement of theannulus volume (301, 401), where the mentioned valves (302, 402, 305,405) and the at least one pressure instrument (303, 403, 316, 416) isarranged to read off and control the pressurization anddepressurization, and where pressure measurements in the referencevolume (304, 404) and in the annulus volume (301, 401) prior to andafter depressurization, together with the reference volume (304, 404)dimension/size are used for calculation of the annulus free volume. 2.Arrangement according to claim 1, where the calculation of the annulusfree volume is performed by equations according to claim
 17. 3.Arrangement according to claim 1, where the arrangement furthercomprises: at least one logical unit (308, 408) for read off and controlof the mentioned valves (302, 402, 305, 405) and the at least onepressure instrument (303, 403, 316, 416) for pressurization anddepressurization, where the logical unit (308, 408) further is arrangedto use the pressure measurements in the reference volume (304, 404) andthe annulus volume (301, 401) prior to and after depressurization andthe dimension of the reference volume (304, 404) for calculation of theannulus free volume.
 4. Arrangement according to claim 1, where thearrangement further comprises: at least one gas container (410) for gasfeed for pressurization, a pressure regulator (412) connected to the gascontainer (410) for pressure regulation of the gas feed, and at leastone valve (415) to open and close for pressurization of the at least oneannulus volume (401).
 5. Arrangement according to claim 4, where thearrangement further comprises: a pressure instrument (411) connected tothe gas container (410) to measure the pressure in this gas container,at least one pressure instrument (414) connected to the pressureregulator (412) to indicate the regulator settings, where the mentionedpressure instruments (411, 414) are further connected to the logicalunit (408) for control and read off.
 6. Arrangement according to claim1, where the arrangement further comprises: at least one pressureinstrument (303, 403) for pressure measurement in the reference volume(304, 404).
 7. Arrangement according to claim 3, where the arrangementcomprises: at least one pressure relief valve (413) for regulation ofgas from the gas container (410).
 8. Arrangement according to claim 1,where the arrangement comprises: at least one pressure relief valve(307, 407) for regulation of gas from the at least one annulus volume(301, 401) to at least one outlet (306, 406).
 9. Arrangement accordingto claim 4 where gas feed is from the mentioned gas container (410) oran internal gas container.
 10. A method for determination of annulusfree volume (301, 401) in a pipe, characterized by acts comprising:establishing a pressure difference between the annulus volume (301, 401)and the reference volume (304, 404), performing a number ofdepressurizations from the annulus volume (301, 401) to the referencevolume (304, 404), where the number of depressurizations is one or more,measuring the annulus volume pressure prior to and after the number ofdepressurizations, and calculating the annulus free volume based on themeasured pressure in the annulus volume (301, 401) prior to and afterthe number of depressurizations, pressure in the reference volume (304,404) prior to and after each of the number of depressurizations and thereference volume dimension/size.
 11. Method according to claim 10,further comprising: establishing a predefined pressure in the referencevolume prior to each of the number of depressurizations, and measuringpressure in the reference volume after each of the number ofdepressurizations.
 12. Method according to claim 10, wherein theestablishing pressure difference comprises gas diffusion to the annulusvolume for pressurization.
 13. Method according to claim 10, wherein theestablishing pressure difference comprise gas feed from a gas containerto the annulus volume for pressurization.
 14. Method according to claim10, wherein the establishing pressure difference comprises reduction ofthe pressure in the reference volume.
 15. Method according to claim 10,further comprising: after establishing of the pressure difference,determining whether the measured pressure in the annulus volume hasreached a high pressure limit, followed by maintaining the pressure atthe high pressure limit for a selected period of time, and determiningwhether the measured pressure in the annulus volume after the selectedperiod of time is within a predetermined accuracy level of the highpressure limit followed by further pressurization if the measuredpressure is not within the predetermined accuracy of the high pressurelimit, and further comprising depressurization from the annulus volumeif the pressure is stabilized.
 16. Method according to claim 10, furthercomprising: after depressurization, determining whether the measuredpressure in the annulus volume has reached a low pressure limit, andmaintaining the annulus volume pressure for a selected period of timewhen the measured pressure in the annulus volume has reached the lowpressure limit after the selected period of time, determining whetherthe measured pressure in the annulus volume is within a predeterminedaccuracy of the low pressure limit, followed by further depressurizationof the measured pressure in the annulus volume is not within thepredetermined accuracy, and followed by calculation of annulus freevolume if the pressure is stabilized.
 17. Method according to claim 10,where the calculation of the annulus free volume is performed by use ofthe following equations:$V = \frac{V_{out}}{\left( {P_{a\; 1} - P_{a\; 2}} \right)}$$V_{at} = {\sum\limits_{i = 1}^{i = n}\; {V_{ref}\left( {P_{{ref}\; 1i} - P_{{ref}\; 2i}} \right)}}$Where: V is annulus free volume, V_(out) is accumulated volume duringdepressurization, V_(ref) is reference volume, P_(a1) is annuluspressure prior to the number of depressurizations (i=1) P_(a2) isannulus pressure after the number of depressurizations (i=n) P_(ref1) ispressure in the reference volume prior to depressurization number i tothe outlet, P_(ref2) is pressure in the reference volume afterdepressurization number i to the outlet, and n is the number ofdepressurization cycles.
 18. Method according to claim 10, wherein theaccumulated volume calculation is performed by at least one logical unit(308, 408) based on data received from emptying the reference volume(304, 404) through a first vale or a second valve (305, 405) at leastone time until the low pressure limit in the annulus volume is reached.19. Method according to claim 10, wherein establishing the pressuredifference comprises pressurization performed by opening valve (302) andclosing the first valve (305) for use of diffused gas, or adjustment ofpressure regulator (412), opening of the second valve (415) and closingof the second valve (405) if the pressure difference is established foruse of gas feed from a gas container.
 20. Method according to claim 15,wherein the selected time period is zero for use of diffusion, or thepressure is maintained for a predetermined period of time until thepressure is stabilized if gas feed from a gas container (410) is used.21. Method according to claim 15, wherein when it is determined that thepressure is stabilized at a predefined high pressure limit, performing apressure measurement by use of pressure instrument (303) if diffused gasis utilized, or by closing a third valve (415) and measuring pressure byuse of pressure instrument (403) if gas feed is utilized.
 22. Methodaccording to claim 15, wherein emptying of the reference volume (304,404) is performed at least one time by closing a fourth and a fifthvalve (302, 402) and opening the first or second valve (305, 405) untilthe low pressure limit in the annulus volume (301, 401) is reached. 23.Method according to claim 15, wherein it is determined that the pressureis stabilized at the predefined low pressure limit, performing pressuremeasurement by use of pressure instruments (303, 403, 316, 416). 24.Method according to claim 10, comprising at least one of the followingactions. calculations corrected for diffusion, calculations correctedfor temperature differences, and control of one or more components ofthe arrangement (309, 409) according to one of the claims 1-9. 25.Method according to claim 10, where the following actions are performedby a logical unit (308, 408): control at least one valve (302, 402) forpressurization and depressurization of at least one annulus volume (301,401), read off at least one pressure instrument (303, 403) for pressuremeasurement of at least one reference volume, control at least one valve(305, 405) for pressurization and depressurization of at least onereference volume (304, 404), and read off at least one pressureinstrument (303, 403, 316, 416) for pressure measurement of at least oneannulus volume (301, 401).
 26. Method according to claim 10, wherein thefollowing actions are performed by a logical unit (308, 408): reading atleast one pressure instrument (411) for pressure measurement of at leastone gas container (410), controlling at least one pressure regulator(412) for regulation of pressure of the gas feed from the gas container(410), reading at least one pressure instrument (414) to check thesettings of the pressure regulator (412), and controlling at least onevalve (415) for pressurization and depressurization of at least one gascontainer (410).
 27. Method according to claim 10, further comprising atleast one of the following actions: comparing calculated annulus freevolume to one or more previously calculated annulus free volumes forintegrity evaluation, comparing accumulated volume per time to a normalcondition, comparing accumulated volume per time to an historical trend,using calculated annulus free volume for calculation of the pipe'sremaining lifetime, saving and presenting measured and calculatedvalues, and triggering one or more alarms if critical values aredetermined.
 28. Method according to claim 10, wherein the method isperformed performed continuously or at regular time intervals.